1. The Field of the Invention
This invention relates to electrical terminations and, more particularly, to novel systems and methods for transferring electrical current from a lead to a thin film.
2. The Background Art
The semiconductor manufacturing industry relies on numerous processes. Many of these processes require transportation and heating of de-ionized (DI) water, acids, and other chemicals. By clean or ultra-pure is meant that gases or liquids cannot leach into, enter, or leave a conduit system to produce contaminants above permissible levels. Whereas other industries may require purities on the order of parts-per-million, the semiconductor industry may require purities on the order of parts-per-trillion.
Chemically clean environments for handling pure de-ionized (DI) water, acids, chemicals, and the like, must be maintained free from contamination. Contamination in a process fluid batch may destroy hundreds of thousands of dollars worth of product. Several difficulties exist in current systems for heating, pumping, and carrying process fluids (e.g., acids, DI water). Leakage into or out of a process fluid conduit must be eliminated. Moreover, leaching and chemical reaction between any contained fluid and the carrying conduits must be eliminated.
Elevated temperatures in semiconductor processing are often over 100 C., and often sustainable over 120 C. In certain instances, temperatures as high as 180 C. may be approached. It is preferred that all process fluid heating and carrying mechanisms virtually remove the possibility of contact with any metals, regardless of the ostensibly non-reactive natures of such metals. It is desirable to prevent process fluid contamination, even in the event of a catastrophic failure of any element of a heating, transfer, or conduit system.
Conventional immersion heaters place a heating element, typically sheathed in a coating, directly into the process fluid. The heating element and process fluid are then contained within a conduit. Temperature transients in immersion heaters may overheat a sheath up to a melting (failure) point. A failure of a sheath may directly result in metallic or other contamination of the process fluid. Meanwhile, temperature transients in radiant heaters may fracture a rigid conduit.
A heating alternative is needed that does not have the risks associated with conventional radiant and immersion-heating elements. A system is needed that is both durable and responsive for heating process fluids. Failure that may result in fluid contamination is an unacceptable risk.
It is a primary object of the present invention to provide a heater for handling process fluids at elevated temperatures in the range of 0 C. to 180 C. It is an object of the invention to provide a heater having electrical resistance in close proximity to a process fluid for heating by conduction and convection without exposing process fluids to contamination, even if electrical failures or melting of conductive paths should occur within a heater.
Consistent with the foregoing objects, and in accordance with the invention as embodied and broadly described herein, a method and apparatus are disclosed in one embodiment of the present invention as including a heater comprising a substrate. The substrate may be formed a material having suitable strength, heat transfer characteristics, non-reactivity, and coating adherence. The substrate may function to separate a heating element from the fluid to be heated. The substrate may have any suitable shape which may promote efficient heat transfer to the fluid passing there across. In certain embodiment, the substrate may be formed as a conduit to transfer the fluid.
In one embodiment, the substrate is one or more tubes of quartz. In such an embodiment, the tubes may be abutted end-to-end with an adapter (e.g. fluorocarbon fitting) fitted to transfer the fluids between two tubes in a series. One pass or passage, comprising one or more tubes of quartz in a series, may be fitted on each end to a manifold (e.g. header/footer) comprised of a fluorocarbon material properly sealed for passing liquid into and out of the individual passage.
Individual tubes or conduits may improve the temperature distribution therein by altering the internal boundary layer of heated fluids passing therethrough. In one embodiment, a baffle tube, within the outer tube, may have a plug serving to center the baffle in the heating tube. The plug may restrict flow, such that the fluid inside the baffle does not change dramatically. Thus an annular flow between the baffle tube and the outer heating tube may maintain a high Reynolds number in the flow, enhancing the Nusselt number, heat transfer coefficient and so forth. Moreover, the temperature distribution may be rendered nearer to a constant value across the annulus, rather than running with a cold, laminar core.
In one embodiment, a heater may be manufactured by depositing, plating, or otherwise adhering a resistive coating or layer to a surface of the substrate. The resistive coating may be any material having a proper balance of conductivity, resistivity, and adherence. In certain embodiments, the substrate surface may be roughened or otherwise prepared to promote adherence of the resistive coating thereto. In one embodiment, electroless nickel may be plated on a roughened (textured) surface of the substrate.
A resistive, conductive coating may extend along any selected length of the substrate. The resistive coating may be configured to connect in series or to multi-phase power along the length of a single substrate. In one embodiment, a quartz tube may be roughened, etched, dipped, coated, and protectively coated. The quartz tube need not be heated to sinter the conductive layer. The conductive coating may be plated as a continuous ribbon of well-adhered, resistive, conducting, metallic material.
The electrical length of the heated portion (i.e. the area coated with the resistive coating) may be adjusted by application of an end coating for distributing current. Electrical current may be applied to the end the coating or directly to the resistive coating by any suitable termination. In selected embodiments, a electrical lead may be soldered to directly to the end coating. In other embodiments, a conductor may be applied against the end coating. The conductor may be formed of multiple conductive strands. The strands may be formed to distribute mechanical and electrical loads substantially evenly across the entire termination zone. The size of the termination zone area may be selected to provide an acceptable current density such that thermal and mechanical loads do not become excessive at any one location. In one embodiment, the conductor may be a braided strap. A clamp may urged the conductor against the end coating, resistive coating, or some other interface layer applied to the substrate. The clamp may maintain the conductor against the underlying surface, while accommodating expansion with temperature, without harming mechanical bonds between the resistive coating and the substrate.